Because the Italians generally show a little more design taste than to butcher a three-star design to accommodate a fourth.
If you look at the logo (not the best copy, but you get the idea):
your only choices would be to jam another star above the FIGC swirl -- and thus create a design imbalance with multiple stars per tricolor strip. If they had three stars running across the top, they could have pulled a Brazil. But in this case, each star is married to each of the tricolor bands -- it fails.
Mark my words. The design is f*%#$ed and the Italians will redo it for four stars.